1. Field of the Invention
This invention relates to a silver halide photographic emulsion spectrally sensitized with at least two sensitizing dyes having supersensitizing effects on each other, and more specifically, to a silver halide photographic emulsion having increased spectral sensitivity in the red wavelength region.
One well-known technique of producing photographic materials is a spectrally sensitizing process by which the sensitive wavelength region of a silver halide photographic emulsion is broadened to a longer wavelength region by adding a certain kind of cyanine dye. It is generally known that the spectral sensitivity of a silver halide photographic emulsion is affected by the chemical structure of the sensitizing dye and the various characteristics of the emulsion such as the halogen composition of the silver halide, the crystal habit, the crystal system, the silver ion concentration of the hydrogen ion concentration, and also by photographic additives present in the emulsion, such as stabilizers, anti-foggants, coating assistants, precipitating agents, or color couplers.
Generally, only one sensitizing dye is used to sensitize a photographic emulsion to a specific spectral wavelength region. When such sensitizing dyes are used in combination, the sensitivity obtained is often lower than that obtained by using the sensitizing dyes individually. However, in some special cases, the spectral sensitivity obtained with a combination of two or more sensitizing dyes markedly increases. This kind of sensitization is known as "supersensitization". A precise selection of sensitizing dyes used in combination, however, is required since a slight difference in chemical structure can markedly affect the supersensitizing effect of the dyes employed. Accordingly, appropriate combinations of sensitizing dyes having supersensitizing effects are difficult to predict from consideration of their chemical structural formulas only.
Generally, the sensitizing effect of a dye on a certain emulsion can be varied by changing the emulsion characteristics. For example, the sensitizing activity can be increased by increasing the silver ion concentration, or by decreasing the hydrogen ion concentration, or by employing both of these techniques. The sensitizing activity can, therefore, be increased by immersing a film coated with the spectrally sensitized emulsion in water or an aqueous solution of ammonia. The above method by which the sensitivity of a sensitized emulsion is changed by increasing the silver ion concentration or decreasing the hydrogen ion concentration or by both of these techniques is usually called "hypersensitization". Hypersensitized emulsions generally have a short storage life.
When supersensitization is applied to a silver halide photograhic emulsion, the sensitizing dye must not have adverse interactions on photographic additives other than the sensitizing dyes, and stable photographic properties must be maintained even during the storage of the photographic materials. A further requirement of the sensitizing dyes used is that no "residual coloration" due to the sensitizing dyes must remain in the photographic materials after processing. This requirement is especially important when the photograhic materials are processed rapidly within short periods of time (usually several seconds to up to about 1 minute).
In order to obtain excellent color reproducibility in a color photographic material, the red sensitive layer preferably does not have a high sensitivity at too long a wavelength, for example, at wavelengths longer than 660 nm (the wavelength at which sensitization is maximum), and preferably does not have a sensitivity at too short a wavelength, for example, at wavelengths shorter than 580 nm (at which sensitization is maximum). According to spectral sensitizing techniques, it is difficult to increase the sensitivity in a wavelength region not exceeding about 630 nm (the wavelength at which sensitization is maximum). Among all, it is particularly difficult to increase the sensitivity in the wavelength region ranging from about 580 nm to 630 nm, and therefore, to solve this problem is one of the important subjects in the art.